diff --git a/backend/app/channels/dingtalk.py b/backend/app/channels/dingtalk.py
index af8beef19..3e4254d6b 100644
--- a/backend/app/channels/dingtalk.py
+++ b/backend/app/channels/dingtalk.py
@@ -416,6 +416,19 @@ class DingTalkChannel(Channel):
             # chat_id uses conversation_id for groups, sender_staff_id for P2P
             chat_id = conversation_id if conversation_type == _CONVERSATION_TYPE_GROUP else sender_staff_id
 
+            # An empty chat_id does not identify a conversation, and ChannelStore keys on it:
+            # a P2P message with no sender keys to the literal "dingtalk:", so every such
+            # message from every user shares one thread and one history. Mirrors the guard the
+            # WeChat channel already applies (wechat.py, `if not chat_id: return`) and the one
+            # this file's own bind path applies before persisting a connection.
+            if not chat_id:
+                logger.warning(
+                    "[DingTalk] ignoring message with no conversation identity: conv_type=%s, msg_id=%s",
+                    conversation_type,
+                    msg_id,
+                )
+                return
+
             inbound = self._make_inbound(
                 chat_id=chat_id,
                 user_id=sender_staff_id,
diff --git a/backend/tests/test_dingtalk_channel.py b/backend/tests/test_dingtalk_channel.py
index f2a65c625..6138d62dc 100644
--- a/backend/tests/test_dingtalk_channel.py
+++ b/backend/tests/test_dingtalk_channel.py
@@ -334,6 +334,102 @@ class TestOnChatbotMessage:
 
         _run(go())
 
+    @pytest.mark.parametrize("sender_staff_id", ["", None])
+    def test_p2p_message_without_sender_is_dropped(self, sender_staff_id):
+        """A P2P chat_id *is* the sender, so an empty one keys every user to one thread.
+
+        ``ChannelStore._key`` builds ``f"{channel}:{chat_id}"`` for a topic-less conversation,
+        so publishing this would put every senderless P2P message under the literal
+        ``"dingtalk:"`` — one shared thread, one shared history, across users.
+        """
+
+        async def go():
+            bus = MessageBus()
+            bus.publish_inbound = AsyncMock()
+            channel = DingTalkChannel(bus, config={})
+            channel._client_id = "test_key"
+            channel._main_loop = asyncio.get_event_loop()
+            channel._running = True
+
+            msg = _make_chatbot_message(
+                text="hello",
+                conversation_type=_CONVERSATION_TYPE_P2P,
+                sender_staff_id=sender_staff_id,
+                message_id="msg_no_sender",
+            )
+
+            channel._send_running_reply = AsyncMock()
+            channel._on_chatbot_message(msg)
+
+            await asyncio.sleep(0.1)
+
+            bus.publish_inbound.assert_not_awaited()
+
+        _run(go())
+
+    def test_group_message_without_conversation_id_is_dropped(self):
+        """The group route reaches the same degenerate key from the other side."""
+
+        async def go():
+            bus = MessageBus()
+            bus.publish_inbound = AsyncMock()
+            channel = DingTalkChannel(bus, config={})
+            channel._client_id = "test_key"
+            channel._main_loop = asyncio.get_event_loop()
+            channel._running = True
+
+            msg = _make_chatbot_message(
+                text="hello group",
+                conversation_type=_CONVERSATION_TYPE_GROUP,
+                sender_staff_id="user_002",
+                conversation_id="",
+                message_id="msg_no_conv",
+            )
+
+            channel._send_running_reply = AsyncMock()
+            channel._on_chatbot_message(msg)
+
+            await asyncio.sleep(0.1)
+
+            bus.publish_inbound.assert_not_awaited()
+
+        _run(go())
+
+    def test_group_message_with_unknown_sender_is_still_delivered(self):
+        """Reverse anchor: the guard is on the conversation identity, not on the sender.
+
+        A group message identifies its conversation through ``conversation_id``, so an
+        unknown sender must not cost the whole message.
+        """
+
+        async def go():
+            bus = MessageBus()
+            bus.publish_inbound = AsyncMock()
+            channel = DingTalkChannel(bus, config={})
+            channel._client_id = "test_key"
+            channel._main_loop = asyncio.get_event_loop()
+            channel._running = True
+
+            msg = _make_chatbot_message(
+                text="hello group",
+                conversation_type=_CONVERSATION_TYPE_GROUP,
+                sender_staff_id="",
+                conversation_id="conv_group_003",
+                message_id="msg_group_003",
+            )
+
+            channel._send_running_reply = AsyncMock()
+            channel._on_chatbot_message(msg)
+
+            await asyncio.sleep(0.1)
+
+            bus.publish_inbound.assert_awaited_once()
+            inbound = bus.publish_inbound.await_args.args[0]
+            assert inbound.chat_id == "conv_group_003"
+            assert inbound.topic_id == "msg_group_003"
+
+        _run(go())
+
     def test_command_classified_correctly(self):
         async def go():
             bus = MessageBus()